President Joe Biden and first lady Jill Biden arrived at the Sept. 11 memorial services at Shanksville, Pennsylvania.

The president laid a wreath at the Flight 93 National Memorial and silently reflected in front of the wall with the names of the 40 victims.

The Bidens walked the grounds of the memorial and spoke with family members of the victims who were in attendance.

Vice President Kamala Harris spoke at the Sept. 11 memorial service at Shanksville, Pennsylvania and reflected on the heroism of the passengers who fought the hijackers.

"What happened on Flight 93 told us then, and it still tells us so much about the courage of those on board who gave everything they possibly could, about the resolve of the first responders who risked everything and about the resilience of the American people," she said.

Harris said Sept. 11, 2001 showed the power of unity among strangers and that sentiment is still powerful 20 years later. The vice president called on Americans to continue that camaraderie.

"In a time of outright terror, we turned toward each other. In the face of a stranger, we saw a neighbor and a friend. That time reminded us the significance and the strength of our unity as Americans, and that it is possible in America," she said.

Former President George W. Bush spoke at the Sept. 11 memorial at Shanksville, Pennsylvania, where United Flight 93 crashed after passengers fought with and overcame the hijackers.

Bush reflected on the bravery of those passengers amid the carnage taking place throughout the country.

"Here, the intended targets became the instruments of rescue, and many who are now alive owe a vast, unconscious debt to the defiance displayed  in the skies above this field," he said.

Bush spoke about the unity and heroism that took place in the days after the attacks, the sights of people grabbing another person's hand and "rally to the cause of one another."

He also warned about the still lingering threat of terrorism, both foreign and domestic.

Bush said there is "little cultural overlap between violent extremists abroad and violent extremists at home."

"They are children of the same foul spirit and it's our continuing duty to confront them," he said.

Bush also commended the country's veterans and armed service members who have served since the attacks.

"Nothing that has followed, nothing, can tarnish your honor or diminish your accomplishments. To you and the honored dead, our country is forever grateful," he said.

NATO marked the 20th anniversary of the Sept. 11 attacks on Saturday with a commemoration ceremony in Brussels.

Secretary General Jens Stoltenberg was joined by the U.S. Mission to NATO's Chargé d'Affaires Douglas D. Jones at the ceremony.

A wreath was laid and those in attendance fell silent as they paid their respects to those who died in the attacks

-ABC News' Guy Davies